1. Field of the Invention
The invention relates to the field of gamut mapping for color devices, and more particularly relates to gamut mapping between a source device and a destination device, the destination device having a destination device black point with a hue, and the source and destination devices being respectively characterized by a source gamut shell and a destination gamut shell.
2. Description of the Related Art
Gamut mapping is used to reproduce an image rendered by a source device on a destination device, since the source device and the destination device typically have different gamut boundaries with respect to each other. In other words, the gamut of colors that can be reproduced by the source device is typically not the same as the gamut of colors that can be reproduced by the destination device. Gamut mapping can be used to render the image from within the source device gamut boundary to within the destination device gamut boundary, so as to more accurately reproduce the color image on the destination device.
Gamut mapping of image data from one gamut to another is typically performed using a gamut mapping algorithm (GMA), which is a particular method of mapping color data between gamuts. In SGCK and similar GMAs, lightness compression and chroma compression are performed, usually in two separate steps. In performing lightness and chroma compression, a source gamut shell and a destination gamut shell, which respectively correspond to the borders of the gamuts for the source and destination devices, are typically used.
Lightness compression takes a source lightness range into a destination lightness range, with the source and destination lightness ranges being defined in terms of a neutral maximum lightness and a neutral minimum lightness. The concept of “neutral” is defined relative to the color appearance space used. For example, in the color appearance space of CAM02 with Jab coordinates, a color is neutral if a=b=0. It should be noted that a color may be neutral in one color appearance space and not neutral in another, and the same can be said even if the same color appearance model (CAM) is used, but with different initialization parameters for the CAM.
One use of the neutral maximum and minimum lightnesses is for initialization of a lightness scaler, which is typically applied to the input colors as well as the source gamut shell. The application of the lightness scaler to the source gamut shell brings the source gamut shell to the same place as the destination gamut shell.
One example of bringing the source gamut shell to the destination gamut shell is depicted in FIG. 1. In this drawing, Jmax represents the neutral maximum lightness while Jmin represents the neutral minimum lightness. As can be seen in FIG. 1, the source gamut shell and the destination gamut shell match at Jmax (i.e., the neutral maximum lightness), and also match at Jmin (i.e., the neutral minimum lightness). In this regard, the chroma compression algorithm, which is typically performed after lightness compression, relies on such matching geometry.
However, use of the neutral maximum and minimum lightnesses for initializing lightness scalers is not without problems. For example, when a printer is used as the destination device, the color with highest density can typically be obtained by outputting 100% inks, or by using maximum possible coverage if Gray Component Replacement (GCR)/ink limiting is in effect. However, this color does not usually correspond with the neutral minimum lightness in the color appearance space of the printer. One such example is shown in FIG. 2, which depicts that the device black point for the printer has a lower lightness than the printer's neutral minimum lightness in color appearance space.
As such, if the neutral minimum lightness of the destination device is used as the minimum lightness (i.e., black), the resulting lightness scaler will likely map to a minimum lightness that is not the highest density achievable by the printer. Furthermore, in a case of reproducing images from a monitor to a printer, the monitor black (i.e., R=G=B=0) will likely not be printed with the highest density possible by the printer. It is therefore likely that the image quality of the printer will lack depth and contrast when printing dark, almost neutral colors.
Accordingly, it is desirable for a GMA to print dark, almost neutral colors with the highest possible density for the destination device.